1. Field of the Invention
This invention relates to electromagnetic flow meters, and more particularly to an electromagnetic flow meter in which pulse width modulation is employed to improve measurement accuracy.
2. Description of the Prior Art
Conventional electromagnetic flow meters employ a DC excitement system in which the exciting coil of a flow rate detector is excited by DC current, or an AC excitement system in which the exciting coil of a flow rate detector is excited by AC current.
In each of the two systems, a magnetic field is formed perpendicular to the direction of the flow of a fluid to be measured. An electromotive force is then induced by the fluid flow in a direction perpendicular to both the direction of the fluid flow and the direction of the magnetic field. The induced electromotive force is detected by an electrode, and the flow rate of the fluid is thereby measured in proportion to the electromotive force.
However, the former DC excitement system is disadvantageous in that electrode polarization is caused by electrolytic components included in the fluid to be measured, which affects the flow rate measurement, and also it is difficult to amplify a very low DC voltage.
On the other hand, the latter AC excitement system is relatively advantageous since the flow rate detector is then not affected by polarization occurring around the electrode, and the signal can be readily amplified. However, this system is still disadvantageous in that the flow rate measurement involves errors because 90.degree.-shifted noise is caused by transformer action regardless of the flow rate, and in addition in-phase noise is caused by the eddy current of an iron core.
In order to eliminate the above-described difficulties accompanying the conventional flow meter, an electromagnetic flow meter employing square wave current to carry out the excitement has been proposed.
In general, in this electromagnetic flow meter, the exciting coil of the flow rate detector is energized by square wave current, and the instantaneous value of the voltage induced by energizing the exciting coil is sampled and held in order to convert the induced voltage into an electrical signal proportional to the flow rate.
However, this excitement system utilizing square wave current is disadvantageous in that if a noise pulse is superposed on the induced voltage at the time of sampling, the flow rate measurement value including this noise is obtained, and therefore the resultant flow rate measurement value is greatly deviated from the true value. PG,4